1. Field of the Invention
The present invention relates to a technique for a display device, and more specifically to a display device including a means for correcting a variation in elements resulting mainly from a change in temperature.
2. Description of the Related Art
In recent years, the development of a display device for displaying an image has been progressed. As the display device, a liquid crystal display device for displaying an image using a liquid crystal element has been widely used for a display screen of a mobile telephone by taking advantages of a high image quality, a thin form, a light weight, and the like.
On the other hand, in recent years, the development of a display device using a light emitting element has been also progressed. The display device using the light emitting element has features such as a high response speed, superior moving picture display, and a wide viewing characteristic in addition to an advantage of an existing liquid crystal display device. Therefore, the display device using the light emitting element has been noted as a next-generation compact mobile flat panel display capable of using moving picture contents.
The light emitting element contains a wide range of materials such as an organic material, an inorganic material, a thin film material, a bulk material, or a dispersion material. In those materials, as a typical light emitting element, there is an organic light emitting diode (OLED) mainly containing an organic material. The light emitting element has a structure in which an anode, a cathode, and a light emitting layer sandwiched between the anode and the cathode are provided. The light emitting layer contains one or plural materials selected from the above-mentioned materials. In general, a response speed of a material composing the light emitting layer is higher than those of a liquid crystal and the like. Therefore, time gradation method is suitable.
In the display device, a plurality of pixels each having a light emitting element and at least two transistors are provided. In each of the pixels, a transistor connected in series with the light emitting element (hereinafter indicated as a driving transistor) has a function of controlling light emission of the light emitting element. When a gate-source voltage (hereinafter indicated as VGS) of the driving transistor and a source-drain voltage (hereinafter indicated as VDS) thereof are changed as appropriate, the driving transistor can be operated in a saturation region or in a linear region.
When the driving transistor is operated in the saturation region (|VGS-Vth|<|VDS|), the amount of current flowing between both electrodes of the light emitting element is greatly dependent on a change in |VGS| of the driving transistor but hardly dependent on a change in |VDS|. A driving method of operating the driving transistor in the saturation region is called constant current drive. FIG. 10A is a schematic view of a pixel to which the constant current drive is applied. In the constant current drive, the gate electrode of the driving transistor is controlled to allow the necessary amount of current to flow into the light emitting element. In other words, the driving transistor is used as a voltage control current source and set such that a constant current flows between a power source line and the light emitting element.
On the other hand, when the driving transistor is operated in the linear region (|VGS-Vth|>|VDS|), the amount of current flowing between both electrodes of the light emitting element is greatly dependent on both values of |VGS| and |VDS|. A driving method of operating the driving transistor in the linear region is called constant voltage drive. FIG. 10B is a schematic view of a pixel to which the constant voltage drive is applied. In the constant voltage drive, the driving transistor is used as a switch, and the power source line and the light emitting element are shorted if necessary, thereby allowing a current to flow into the light emitting element.
The light emitting element has a property in which a resistance value (internal resistance value) is changed according to a change in temperature. More specifically, in the case where a room temperature is assumed to be a normal temperature, the light emitting element has the following property. When a temperature becomes higher than the normal temperature, the resistance value is reduced. On the other hand, when a temperature becomes lower than the normal temperature, the resistance value is increased. A current value flowing between both electrodes of the light emitting element is inversely proportional to the resistance value. Therefore, when the resistance value is increased, the current value is reduced. When the resistance value is reduced, the current value is increased.
FIG. 9 is a graph of voltage-current characteristic versus temperature in the light emitting element. As is apparent from the graph, even if the same voltage value is applied between both electrodes of the light emitting element, the current value depends on a temperature at a time when the display device is used (hereinafter indicated as an environmental temperature). In other words, the current value is varied according to the environmental temperature, thereby changing the brightness of the light emitting element. Therefore, an accurate gradation representation becomes difficult, so that this becomes one of the factors which impair the reliability of the display device.